Spectral sensitization is an important and essential technique for production of light-sensitive materials having high sensitivity and excellent color reproduction. To provide light-sensitive materials of high sensitivity, various spectral sensitizers have been developed, as well as techniques such as supersensitization and an adding method thereof. Spectral sensitizers act to absorb light in the long wavelength region to which an unsensitized silver halide photographic emulsion does not substantially absorbes and to transmit the light absorbing electron and/or light absorbing energy to the silver halide. The increase of the light captured through such spectral sensitizers advantageously contributes to an increase of photographic sensitivity. For this reason, it is desired to develop spectral sensitizers having a high coefficient of light absorbance and to increase the amount of light captured by increasing the amount of the spectral sensitizers added to a silver halide emulsion.
Attempts have been made to develop a method for adding increased amounts of spectral sensitizers and also a method for preparation of improved silver halide emulsions.
For example, Thomas L. Penner and P. B. Gilman, Jr., Photographic Science and Engineering, Vol. 20 (3), pp. 97-106 (1976) disclose a method in which a large amount of two types of spectral sensitizers having a suitable electric potential relationship is adsorbed in a laminated form onto silver halide crystals, thereby increasing the amount of light captured by the spectral sensitizers and also preventing desensitization due to the addition of a large amount of the spectral sensitizers.
In addition, methods to improve silver halide grains themselves have been attempted. One of the methods is to use tabular silver halide grains having an increased specific surface area, as described in Japanese patent application (OPI) Nos. 113926/83, 113927/83, 113926/83, 113930/83, 113934/83, 111934/83, 95337/83, 108528/83, 108526/83, (the term "OPI" as used herein means a "published unexamined Japanese patent application").
C. R. Berry and D. C. Skillman, J. Appl. Phys., Vol. 35, pp. 2165-2169 (1964) disclose silver halide grains in which AgCl is epitaxially grown on the silver bromide grain surface by adding an equal atomic equivalent of a KCl solution to an aqueous suspension of AgBr crystals and then adding thereto a somewhat smaller amount of a AgNO.sub.3 solution. Japanese patent application (OPI) No. 133540/84 discloses grains in which AgCl is epitaxially grown non-selectively on the silver bromide grain surface by double jet addition at 40.degree. C. and a pAg of 7.2.
Although the above grains do not have an increased specific surface area, the specific surface area can be increased by the method of preparation of the grains.
In all of the above grains, however, the size of a protrusion epitaxially grown on the grain surface is large and/or the number of protrusions per unit area is small. With these grains, therefore, the specific surface area cannot be sufficiently increased in comparison with grains of equal size without protrusions.
Although small protrusions are advantageous with respect to specific surface area, they are quite difficult to prepare in a stabilized manner. In practice, therefore, such grains having a structure suitable for chemical sensitization have not been produced.